It is recognised that mixing can be described as either distributive or dispersive. In a multi-phase material comprising discrete domains of each phase, distributive mixing seeks to change the relative spatial positions of the domains of each phase, whereas dispersive mixing seeks to overcome cohesive forces to alter the size and size distribution of the domains of each phase. Most mixers employ a combination of distributive or dispersive mixing although, depending on the intended application, the balance will alter. For example, a machine for mixing peanuts and raisins will ideally be wholly distributive so as not to damage the things being mixed, whereas a blender/homogeniser will be dispersive.
Many different types of rotor/stator mixer are known. Flow-through stirring reactors such as those disclosed in US 2003/0139543 comprise a vessel with internally mounted mixing elements and are generally distributive in function. The direction of bulk flow within such a mixer is from the inlet port to the outlet port.
Other types of rotor-stator mixer (such as that disclosed in WO 2007/105323 are designed with the intention of forming fine emulsions and are dispersive in character. DE 1557171 discloses a mixer with a plurality of alternately rotating and static, concentric cage-like elements through which the direction of bulk flow is radial.
EP 0048590, EP 0799303 and GB 2118058 describe a known mixer type hereinafter referred to as a “Cavity Transfer Mixer” (CTM). The CTM comprises elements which define confronting surfaces, each having a series of cavities formed therein, in which the surfaces move relatively to each other and in which a liquid material is passed between the surfaces and flows along a pathway successively passing through the cavities in each surface. In FIG. 1 of GB 2118058, the confronting surfaces are the inner surface of a sleeve and the outer surface of a co-axially disposed inner drum. The cavities are arranged so that they overlap, forming sinuous flow paths which change as the drum and the sleeve rotate relative to each other. The type of mixer shown in GB 2118058 has stator and rotor elements with opposed cavities which, as the mixer operates, move past each other across the direction of bulk flow through the mixer. In such CTM-type mixers, primarily distributive mixing is obtained. Shear is applied by the relative movement of the surfaces in a generally perpendicular direction to the bulk flow of material along the mixer. In such a device there is relatively little variation in the cross-sectional area for flow as the material passes axially down the device. Generally, the cross-sectional area for flow (due to the cavities) varies by a factor of less than 3 through the apparatus. Absent the cavities, the “metal to metal” separation between the inner surface of the sleeve and the surface of the drum is essentially constant.
The commercial application of CTMs has been largely restricted to the thermoplastics' conversion industry, where CTM technology originated (see EP 048590). In part this is because established rotor/stator devices, such as “Silverson” mixers, offer some of the benefits and at a significantly lower cost.
In some mixers, such as that described in EP 0434124 a cage-like rotor and stator elements are configured such that the bulk flow must pass through relatively narrow spaces within the mixer. Similar alternation of relatively wide and relatively narrow flow spaces, for the purpose of forming an emulsion, are known from GB 129757. GB 129757 discloses a mixer in which the confronting surfaces are formed between two conical members, located one within the other. The inner conical member is a rotor and has two semicircular, circumferential and horizontally disposed grooves which, together with similar grooves on the confronting surface of the outer conical member define annular mixing chambers between regions of high extensional flow. A further feature of the mixer disclosed in GB 129757 is that the spacing between the confronting surfaces tapers in the direction of bulk flow, such that the normal spacing between the surfaces (i.e. the spacing ignoring the grooves) is reduced in the direction of bulk flow.
GB 129757 and EP 0434124 are not CTM's as the relatively wide spaces within the mixers form annuli and there it little or no alteration of the flow path geometry as the rotor and stator move.
EP 0799303 describes a mixer, hereinafter referred to as a “Controlled Deformation Dynamic Mixer” (CDDM). In common with the CTM, this type of mixer has stator and rotor elements with confronting surfaces having opposed cavities which, as the mixer operates, move past each other across the direction of bulk flow through the mixer. The CDDM is distinguished from the CTM in that material is also subjected to extensional deformation. The extensional flow and efficient dispersive mixing is secured by having confronting surfaces with cavities arranged such that the cross sectional area for bulk flow of the liquid through the mixer successively increases and decreases by a factor of at least 5 through the apparatus. In comparison with the embodiment of the CTM described above, the cavities of the CDDM are generally aligned or slightly offset in an axial direction such that material flowing axially along the confronting surfaces is forced through narrow gaps as well as flowing along and between the cavities. The CDDM combines the distributive mixing performance of the CTM with dispersive mixing performance. Thus, the CDDM is better suited to problems such as reducing the droplet size of an emulsion, where dispersive mixing is essential. As with the CTM disclosed in GB 2118058, the normal spacing of the confronting surfaces (absent the cavities) in the CDDM is constant along the length of the mixer. GB 129757 does not disclose a CDDM mixer because although regions of dispersive extensional flow alternate with distributive mixing zones, the distributive mixing zones are annular and do not have the CTM-like mixing action across the bulk flow through the mixer.
GB 2308076 shows several embodiments of a mixer comprising a co-called “sliding vane” pump. These include both drum/sleeve types where the bulk flow is along the axis of the mixer and mixers in which the flow is radial. Many other types of mixer can be configured either as the drum/sleeve type or the “flat” type. For example DD207104 and GB 2108407 show a mixer comprising two movable confronting surfaces with projecting pins which cause mixing in material flowing in a radial direction between the plates.
Both the CTM and the CDDM can be embodied in a “flat” form where the drum and the sleeve are replaced with a pair of disks mounted for relative rotation and the cavities are provided in the confronting surfaces of the disks. In this modified “flat” form the bulk flow is generally radial.
An important further consideration in certain CDDM designs concerns the relative axial positions of rotor and stator components during operation which are critical to performance. Such relative positions may change by axial displacement of the rotating parts with respect to the static parts and this may compromise critical clearances. Under “normal” operating conditions, such displacement is resisted through thrust bearings, an approach which becomes more difficult at high pressures and mixer speeds.
There are practical limits to the spacing between the confronting surfaces in the CDDM and CTM. As the device is heated, expansion may mean that the rotor/drum expands in a radial direction. The stator/sleeve may expand less as it is better able to lose heat. This can result in a narrowing of the gap between the confronting surfaces and even contact. At high operating speeds, contact between the surfaces can be catastrophic.
Further difficulties arise from the high shear rates which are encountered in mixers with very closely confronting surfaces. High shear rates lead to high shear stress (which is a function of shear rate and viscosity). These shear stresses lead to a high torque (which is related to the shear stress for a given geometry). For a fixed angular velocity of the mixing elements the power consumption is directly related to the torque. Hence mixers which employ high shear rates typically require large power inputs. This is not only adds cost, but can produce unwanted or uncontrolled heating of the material being processed.